Centrifuges having rotors with fixed angle receiving chambers for a sample container (called a fixed angle rotor) are commonly known and manufactured by a number of sources. See generally Centrifugation and Biology in Medical Science by Phillip Scheeler, John Wiley & Sons Publishers, 1981. A fixed angle rotor comprises a rotor body having a generally disc-like shape with a rim portion, or other suitable structure around the circumference of the rotor into which a plurality of receiving chambers are formed. The receiving chambers formed have an angular relationship with the central axis of the rotor body about which the rotor body is rotated. Since the chambers are formed into the structure of the rotor body, the angular relationship which they possess relative to the rotational axis of the rotor is fixed. Each of the receiving chambers can receive a tube or other container containing a sample material which is to undergo centrifugation.
The angular relationship relative to the rotational axis selected for the receiving chambers formed in a rotor is generally between 25.degree. and 45.degree.. The specific angle which the chambers should have is determined by the sample content and the type of molecular separation which is desired to be performed by the centrifuge. Receiving chambers having a small angular relationship, i.e., near 25.degree., with the rotational axis of the rotor will have a greater average and more uniform centrifugal force field applied throughout the space defined by the chamber. This is due to the fact that the change in radial distance between the rotational axis of the rotor and the central axis of the chamber is less than that of greater angular chamber positions, according to the formula for the magnitude of centrifugal force at a point on a rotating element; thus, the more uniform the radius, the more constant the centrifugal force applied and the greater average radius over an elongate cavity, the greater the average centrifugal force applied thereto.
Conversely, receiving chambers having a greater angular relationship with the rotational axis of the rotor will have a more varied centrifugal force field applied along the length of the space defined by the chamber.
In operation, it is more desirable to have a smaller angular relationship of the chamber with the rotational axis of the rotor because in samples having sedimentable material, the sediment is separated from the sample solution more quickly due to the higher average centrifugal force field. However, the reduction of angular relationship of the chamber is limited by the ability of the sedimented material to flow or move along the side of the container in which the sample is contained once removed from solution. The angle at which material begins to accumulate along the side of the sample container during sedimentation in a centrifugal force field, and not flow toward the bottom of the sample container is defined as the angle of repose of that material. It is the angle of repose defined for a specific material which determines the desired angle at which a receiving chamber should be formed in the centrifuge rotor, to perform the desired separation and analysis.
Furthermore, it is often necessary for centrifuge analysis to be performed according to a documented protocol of past studies, which can require a specific angular definition of the receiving chamber in the fixed angle rotor of the centrifuge, for accurate comparative analysis.
In order to deal with a number of different sample solution compositions having various sample materials for separation, it has heretofore been necessary to provide numerous removable rotors for a centrifuge, each rotor having a select angular relationship of the receiving chambers for a sample container formed therein relative to the axis of rotation of the rotor for the cavities. Thus, a change in the composition of the sample to be centrifuged often requires a change in the fixed angle rotor used to obtain correct separation of sedimentable material.